HDLC is a bit-oriented synchronous data link layer protocol that provides both connectionless and connection-oriented services. It uses synchronous transmission in the form of frames with flag fields to delimit frames. There are three types of stations - primary controls the link, secondary operates under control of primary, and combined has features of both. Data can be transferred in normal response mode with an unbalanced configuration or asynchronous balanced mode with a balanced configuration. The frame structure includes address, control, information, and frame check sequence fields. HDLC operates in three phases - initialization, data exchange, and termination.

1. HDLC By : AlaaHamedAbdElhamed Department : IS Section : 4

2. Definition High-Level Data Link Control (HDLC) is a bit-orientedsynchronousdata link layerprotocoldeveloped by the International Organization for Standardization (ISO). HDLC provide both connectionless service and connection oriented services ………….. HDLC uses synchronous transmission. All transmissions are in the form of frames.

3. Station Types Primary : Has the responsibility for controlling the operation of the link. Frames issued by the primary are called commands. Secondary :Operates under the control of the primary station. Frames issued by the primary are called responses Combined : Combine features of both secondary and primary.

4. Data Link Configuration Unbalanced : Consists of one primary and one or more secondary stations and supports both full-duplex and half-duplex transmission Balanced :Consists of two combined stations and supports both full-duplex and half-duplex transmission

5. Data Transfer Mode Normal response mode (NRM):Used with an unbalanced configuration. The primary may initiate data transfer to a secondary, but a secondary may only transmit data in response to a command from the primary. Asynchronous balanced mode (ABM):Used with a balanced configuration. Either combined station may initiate transmission without receiving permission from the other combined station. Asynchronous response mode (ARM):Used with an unbalanced configuration . The secondary may initiate transmission without explicit permission of the primary. The primary still retains responsibility for the line, including initialization , error recovery, and logical disconnection.

6. Frame Structure

7. Flag Fields Flag fields delimit the frame at both ends with the unique pattern 01111110. A single flag may be used as the closing flag for one frame and the opening flag for the next. The pattern 01111110 could be found inside a frame and thus using it as a delimiter will destruct inner structure of the frame thus a method name Bit Stuffing was used in which sender will insert 0 after occurrence of 5 consecutive 1

8. Bit Stuffing With the use of bit stuffing, arbitrary bit patterns can be inserted into the data field of the frame. This property is known as data transparency.

9. Bit Stuffing (Cont)

10. Frame Structure(Cont)

11. Address Field : The address field identifies the secondary station that transmitted or is to receive the frame. This field is not needed for point-to-point links, but is always included for the sake of uniformity. The address field is usually eight bits long but, by prior agreement, an extended format may be used in which the actual address length is a multiple of seven bits. The least significant bit of each octet is 1 or 0, depending on whether it is or is not the last octet of the address field. The single-octet address of 11111111 is interpreted as the all-stations address

12. Control Field : Define three main types of frames : 1)Information frames (I-frames) carry the data to be transmitted for the user. Additionally, flow- and error-control data are piggybacked on an information frame 2)Supervisory frames (S-frames) provide the ARQ mechanism when piggybacking is not used. 3)Unnumbered frames (U-frames) provide supplemental link control functions

13. Control Frames Structure

14. Frame Structure(Cont)

15. Information Field The information field is present only in I-frames and some U-frames. The field can contain any sequence of bits but must consist of an integral number of octets. The length of the information field is variable up to some system-defined maximum.

16. FCS Field The frame check sequence (FCS) is an error-detecting code calculated from the remaining bits of the frame, exclusive of flags. The normal code is the 16-bit CRC CCITT . An optional 32-bit FCS, using CRC-32, may be employed if the frame length or the line reliability dictates this choice.

17. HDLC Operation The operation of HDLC involves three phases. First, one side or another initializes the data link so that frames may be exchanged in an orderly fashion. During this phase, the options that are to be used are agreed upon. After initialization, the two sides exchange user data and the control information to exercise flow and error control. Finally, one of the two sides signals the termination of the operation.

18. HDLC Commands & Responses

19. Data Transfer The N(S) and N(R) fields of the I-frame are sequence numbers that support flow control and error control. N(R) is the acknowledgment for I-frames received; it enables the HDLC module toindicate which number I-frame it expects to receive next. The receive-ready (RR) frame is used to acknowledge the last I-frame received by indicating the next I-frame expected. Receive-not-ready (RNR) acknowledges an I-frame, as with RR, but also asks the peer entity to suspend transmission of Iframes. When the entity that issued RNR is again ready, it sends an RR. REJ initiates the go-back-N ARQ. It indicates that the last I-frame received has been rejected and that retransmission of all I-frames beginning with number N(R) is required. Selective reject (SREJ) is used to request retransmission of just a single frame.

20. How to Disconnect in HLDC Either HDLC module can initiate a disconnect, either on its own initiative if there is some sort of fault, or at the request of its higher-layer user. HDLC issues a disconnect by sending a disconnect (DISC) frame. The other side must accept the disconnect by replying with a UA. Note : See Commands Table to understand DISC and UA

21. Example Command to connect with balanced extended set mode Station1 Satation2 SABM Time Out SABM UA

22. Example (Cont) Disc Command Station1 Station2 Disc UA

23. Example(Cont) Busy State Station 2 Station 1 1,3,0 RNR,4 RR,0,P 1,4,0

24. Example (Cont) Full Duplex Data Exchange N(S) N(R) Station 2 Station 1 1,0,0 1,0,1 1,1,3 1,1,4

25. Thank You